Warplanes and helicopter gunships carried out intense air raids across Syria on Tuesday as fighting raged in and around the capital between regime and rebel forces, a watchdog said.

The raids killed five people in the town of Tabqa in the northern province of Raqa, and six others, including two women and three children, in Sermin in the northwestern province of Idlib, the Syrian Observatory for Human Rights said.

The air force also battered the southern province of Daraa, Homs in central Syria and Aleppo in the north, including the town of Khan al-Assal which rebels claimed to have captured on Monday, the Observatory said.

Five other people were died when artillery rounds struck Douma, a satellite suburb northeast of Damascus, where violence killed another seven people, including two children.

In the south of the capital, troops pounded the Palestinian refugee camp of Yarmuk for the third consecutive day while regime forces and rebels were locked in fierce fighting inside the camp, the Observatory said.

Government forces also shelled neighbourhoods in the south, east and north of Damascus.

On Monday at least 159 people were killed in violence across Syria, said the Observatory, which relies on a network of activists and medics on the ground for its information.

UN told of 13 alleged chemical attacks in Syria: envoy
United Nations, United States / United States (AFP) July 23, 2013 - The United Nations has been told of 13 alleged chemical weapons attacks in Syria, a top UN official said Tuesday ahead of talks between UN experts and the Syrian government.

Robert Serry, UN envoy on the Middle East peace process, told the UN Security Council that UN leader Ban Ki-moon remains "gravely concerned" about reports of the use of chemical arms in the 28-month old conflict.

"The United Nations has received 13 such reports so far," Serry told a Security Council meeting on the Middle East. He said all cases were being "studied".

Serry spoke as the head of a UN investigation into the use of chemical arms in Syria, Ake Sellstrom, and the UN disarmament chief, Angela Kane, arrived in Beirut.

The two officials are expected to go to Damascus on Wednesday to start talks with President Bashar al-Assad's government on securing access to sites in Syria where the arms are said to have been used.

The Syrian government has insisted the UN team only go to the town of Khan al-Assal. The government says opposition rebels used chemical weapons in an attack on the town on March 19 in which at least 26 people, including 16 soldiers, were killed.

The Syrian opposition blames attacks on government forces.

Britain, France and the United States have provided information to the United Nations about attacks which they say Assad forces carried out. Russia has handed over a report on the Khan al-Assal attack which it says showed opposition rebels had fired a shell containing sarin.